Gas present in tanks in LPG/LEG ships at reloading is removed by means of nitrogen or an inert gas. The method which is used presently involves release of the off-gas to atmosphere. This off-gas constitutes a loss of about 250,000 tons of hydrocarbons per year, mainly propane and butane.
NO Patent Application 932417 discloses a method for recovering a portion of this off-gas by condensing the off-gas in heat exchange with liquid nitrogen, whereupon the evaporated nitrogen is used in the removal of gas from the tanks. In relation to the gas removed the consumption of nitrogen with this method is unnecessary high.
U.S. Pat. No. 5,176,002 describes a process for recovery of vapor released during filling of hydrocarbons in storage tanks. Inert gas is supplied to the tank and intermixed with hydrocarbon vapor to form a non-combustible composition. The composition is thereafter subjected to a two-step compression, and a separation process to condense and separate the hydrocarbon vapor from the inert gas. This process involves the use of a closed coolant circuit in the separation/condensation step.